The concept of constant vs. variable capital contrasts with that of fixed vs. circulating capital (used not only by Marx but by David Ricardo and other classical economists). The latter distinction corresponds to the very common distinction in economics, between fixed inputs (and costs) and variable inputs (and costs). It distinguishes inputs from the point of view of their user (the capitalist), in terms of the degree of flexibility that the user has in using them.

On the other hand, constant capital refers to the non-human inputs into production, while variable capital refers to the human input (the hiring of labor power to do labor).

Constant capital can be measured as a stock magnitude, i.e., the total value of means of production in use at a specific point in time. It can also be measured as a flow magnitude, i.e., the total value of raw materials and fixed means of production used up in an accounting period. Which measure is used depends on the purposes and assumptions of one's analysis, for example whether one is interested in the unit-costs of output or in the rate of return on capital invested.

The flow value divided by the stock value provides a measure of the number of rotations of the stock (the speed of turnover or turnover time) in an accounting period. It is strongly related to the actual depreciation rate of fixed capital. Alternatively, the stock value divided by the flow value is what Marx called the "turnover time."

The faster the turnover of constant capital (i.e., the shorter the turnover time), other things being equal, the higher the rate of profit.

Marx calls the constant part of the capital outlay "constant" because according to his labour theory of value, constant capital inputs - once produced, purchased, withdrawn from the market and used to create new products - do not by themselves add new value to output, or increase in value in the production process. Instead, the value of equipment and materials being used in production is conserved and transferred to the new product by living labor.

It is true that the ruling market prices for constant capital inputs could change after they have been bought for use in production, but normally this cannot affect those inputs (having been withdrawn from the market for use in production), only the market valuation of the outputs created from those inputs.

Constant capital contrasts with variable capital, v, the cost incurred in hiring labor power. Marx argues that only living labour creates new value. The higher value of output, compared to input costs, is (other things being equal) attributable to the exploitation of living labor-power only. Variable capital is "variable" because its value changes (varies) within the production process. Although most commentaries on Marx do not acknowledge this, these changes could be both positive or negative. A misapplication of labour, or the devaluation of types of labour activity by the market can mean the loss of part of the capital invested, or all of it. However, Marx does generally assume that labour will accomplish the valorisation of capital.

Critics of Marxian value theory object that labor is not the only source of value-added goods.

Examples of such arguments:

devaluations or revaluations of types of assets in response to changing demand conditions, which are influenced by price inflation. In national accounts and business accounts, for example, the change in the value of inventories held is adjusted for changes in their current market prices, affecting the profit calculation.

Steve Keen[1] argues that "Essentially, Marx reached the result that the means of production cannot generate surplus value by confusing depreciation, or the loss of value by a machine, with value creation" . His argument is, that a machine can add a value to new output in excess of the value of economic depreciation charged[2]

Marxism asserts that capitalist production reduces all production to a combination of commodity inputs to complete the M...M' circuit. However, when one considers human labor to constitute a particular kind of activity among the many necessary to the production of goods, and when one considers that the price of other input factors like raw materials, machines, facilities, land or even other intermediate goods may vary based on the availability of such assets on a market, then the labor commodity is no more or less variable than any other, whether or not it is the sole source of surplus value, and hence profit. The dichotomy "fixed" vs. "variable" is thus a specious mathematicization of the actual "labor" vs. "non-labor" dichotomy.

According to some Marxists, the first two types of objection above cut to the heart of the main dispute between Marx and mainstream economic theory—their different conceptions of value.

For Marx's critics, value, if it exists at all, is a technical feature of economic calculus or is simply another word for the price of a product.

For Marx, however, economic value is a social attribution, which expresses a social relation between people that is specific to certain historical conditions. Inanimate objects can only feature in value relations as tokens of prior human effort, since they are not social beings. Thus, it is not the machine with which new outputs are produced that itself adds value to those outputs, but rather the people operating the machine who permit its value to be conserved and who operate the transfer of part of its value to the new outputs.

Another clarification is that Marx may have used the terms fixed and variable capital to emphasize the idea that the input cost of compensation can be varied by the enterprise, which sets the compensation levels of its workers, whereas the price of the other input factors sold to the company is "fixed," insofar as it is set by external vendors.

Other Marxian economists note that, at any time, most of the stock of objects of value in a society has no actual market price, because those objects are not being traded (i.e. they are withdrawn from the market); they are either being used in production or consumption activities, or else stored for later use. This can be easily verified by striking a ratio between gross product and the estimated total asset wealth of a country in money units.

In other words, this stock of owned objects has, at best, an ideal price which is estimated or hypothesized (the price it might have, if it was traded in the market).

Nobody however will say that because this stock of objects has no actual market price, that it has no value; everybody knows that its exchange value could be expressed in money, within a certain range of probable prices; they may also know approximately that a quantity of one good is "worth" a certain quantity of another good.

This simple insight may help to clarify Marx's concept of value, because it shows that beyond prices there are also economic value relations referring to the changing relationships between objects of value which have no specific, defined or actual market price, and to the social outcome of the interactions between a myriad of prices. In turn, these value relations between objects reflect social relations between people.

The fetish of capital is broken as soon as living labour is withdrawn; then it becomes clear that the constant part of capital produces nothing, and declines in value. Because of its role as a traditional money commodity some individuals give a reference display of this fetish by seeing gold as the only 'real' money, even in the current time when most money is lacking any substantial form whatsoever, even paper.

Critics object however that without the supply of means of production, labour also can produce nothing. That is, separated from means of production, workers are also nothing. This however raises the question of why and how workers come to be separated from the means of production which they have themselves created.

For Marx at least, the answer to this question is not "technical" but purely social, i.e. a matter of property relations which provides capital and its owners with a social power over people. But ownership by itself creates no net addition to new value produced, other than, perhaps, profit from speculation which redistributes existing asset values and claims to them.

As noted above, the distinction between constant and variable capital overlaps with the distinction between fixed capital and circulating capital. Constant capital has both fixed and circulating components: for example, the fixed constant capital would include a factory and the machinery in it, while the circulating constant capital would include the raw materials used and the intermediate inputs produced by the factory.

Variable capital is almost exclusively a component of circulating capital. However, the salaries of some "overhead" employees (who have long-term security from being fired or laid off) are in effect, fixed elements of variable capital.